PubMed · 14826788
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M PIANKA, H C BARANY, C G SMITH. 1951-03-17. Hyperconjugation and light absorption.. https://doi.org/10.1038/167440a0
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An ongoing analysis of the supramolecular self-assembly of disubstituted cycloalkanes has led to the discovery of seven packing patterns built up from hydrogen-bonded homo- and heterochiral chains of racemic molecules, associated in either antiparallel or parallel arrays [Kálmán et al. (2001). Acta Cryst. B57, 539-550]. Two further patterns have been revealed in the close packing of analogous alicyclic beta-amino acids [Fábián et al. (2005). Cryst. Growth Des. 5, 773-782]. Since each pattern is represented by at least one crystal structure, the chemical similarity and crystallographic forms of these crystals have facilitated the recognition that these patterns differ by one or two rotation(s) of the common motifs (e.g. dimers, tetramers, helices etc.), or the whole pattern may rotate through 180 degrees in an oblique unit cell. Such non-crystallographic--with the exception of polymorphism--virtual rotations as a whole may be denoted by the expression morphotropism. According to Kitaigorodskii [(1961), Organic Chemical Crystallography, pp. 222-231. New York: Consultants Bureau], morphotropism is an attempt to keep the packing coefficient above 0.6 whenever there are alternative possibilities for the structures of closely related molecules. It has been found that crystals of stereoisomers are also frequently related by such virtual rotations. Similarly, non-crystallographic rotations effect bridges between homostructural crystals [Kálmán et al. (1993b). Acta Cryst. B49, 1039-1049] and occasionally hallmark the polymorphism of organic compounds [Kálmán et al. (2003) J. Am. Chem. Soc. 125, 34-35]. In polymorphs, however, such rotations really transform one molecule into another in order to achieve a better packing mediated by solvents, temperature etc.
We studied the influence of intermolecular interactions on the first combination band between the OH-stretching and -deformation vibrations of the water that is dissolved in organic and polymeric materials. The water has two types. The first type occurs in the matrix in which water-interactive functional groups are densely distributed. The water is symmetrically hydrogen-bonded through each OH to two functional groups and shows one clear combination band, of which the frequency depends on the strength of the hydrogen bond. The second type occurs in the matrix in which the functional groups are rarely distributed. One OH of this water is hydrogen-bonded to the functional group but the other OH is free. The water shows two combination bands, of which one is sharp and strong at about 5300 cm-1 and the other is weaker and broader, the frequency changing in the 5200-5100 cm-1 range. Thus, the dissolved water sensitively responds to the nature of a water-interactive functional group and its distribution density in a matrix, and shows the first combination band that is characteristic of these factors.
This tutorial review aims at presenting recent contributions dealing with organic chemistry of organophosphorus radicals. The first part briefly lays out the physical organic background of such intermediates. In a second part the use of organophosphorus radicals possessing a P-H bond that can undergo homolytic cleavage as alternative mediators is detailed. The third part is focused on radical additions of phosphorus-centered radicals to unsaturated compounds, an old reaction that is being rejuvenated. Lastly, radical eliminations of phosphorus-centered radical are introduced in the fourth part. Most of the latter are relatively novel reactions, and have never been reviewed previously.